1. Technical Field
The present invention relates generally to ac GTAW welding and, in particular, to a method and apparatus for stabilizing the arc when GTAW welding.
2. Background Art
Gas tungsten arc welding (GTAW), also called tungsten inert gas (TIG) welding, is a well known method for welding metals, in particular aluminum and magnesium. A description of GTAW, including the problems associated with welding aluminum and magnesium, may be found in Welding Handbook, Chapter 3, Gas Tungsten Arc Welding, pages 74-107, 1991, published by the American Welding Society.
It is well known in GTAW to use an ac squarewave power source, which is particularly suited for welding aluminum and magnesium. An ac squarewave power source provides an output having a straight polarity (electrode negative) half-cycle and reverse polarity (electrode positive) half-cycle, with a rapid transition between half cycles.
One such ac squarewave power source is described in U.S. Pat. No. 4,038,515, (the '515 patent) issued Jul. '515 patent provides an ac squarewave welding current output, with adjustable asymmetry or balance, i.e. the duration of the electrode positive and electrode negative half-cycles may be adjusted with respect to one another. As disclosed in the '515 patent it is desirable to provide such asymmetry control. The device disclosed by Risberg includes a four SCR bridge with an inductor in the dc leg, wherein current flows through the inductor in the same direction regardless of the direction of the current through the arc. In this manner, the inductor is able to provide a desirable rapid zero crossing to reduce arc rectification. However, the arc will still occasionally rectify. In particular, extinguishing of the arc (called arc rectification if the arc is extinguished on one of the half cycles on a recurring basis) or arc instability can occur when the current reverses to the electrode positive half cycle because of the difficulty in establishing a "cleaning" arc which has a higher voltage than a "welding" arc.
Another ac square wave power supply is described in U.S. Pat. No. 4,371,776, issued to Winn Feb. 1, 1983 (the '776 patent). The '776 patent also discloses a commutator circuit which is used on the electrode positive half cycle to maintain the arc as the polarity switches from electrode negative to electrode positive. The commutator circuit provides a high voltage on every such transition, not on as need basis.
It is also known to provide a high frequency overlay to assist in arc starting and arc stabilization. Such an overlay consists of applying a high voltage, low current signal at a high frequency across the arc. In the event of arc rectification, the high frequency signal provides a voltage sufficient to maintain or restart the arc on a half-cycle basis. The high frequency overlay may be applied only upon start-up, continuously, or as needed. When applied as needed, arc rectification is sensed and, after rectification has existed for several cycles, the high frequency component is supplied.
There are several problems with these high frequency systems. First, high frequency can provide undesirable noise which adversely affects the operation of control circuits, telecommunication equipment and computer equipment. Second, there has not yet been a practical way to provide an "as needed" high frequency arc stabilization that is able to restart the arc within the completion of one cycle. In other words, there is not a high frequency arc stabilization system that, upon sensing arc rectification, provides the high voltage signal to restart the arc within the half cycle in which the arc was rectified.
Accordingly, it is desirable to have an arc stabilization circuit that operates on an "as needed" basis. Such an arc stabilization system should be capable of providing a high voltage to the arc sufficient to restart the arc, and should be provided to restart the arc before the completion of the half cycle in which rectification occurred. Additionally, the use of high frequency should be avoided. Such a system should preferably be simple and inexpensive to manufacture.